AC NO: AC 20- 76 DATE: 21 Oct 71
ADVISORY
CIRCULAR MAINTENANCE INSPECTION NOTES FOR BOEING B-707/720 SE.RIES AIRCRAFT
DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION
AC NO: 20-76 DATE: 21 Oct 71
ADVISORY
CIRCULAR
DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION SUBJECT: MAINTENANCE INSPECTION NOTES FOR BOEING B 707 /720 SERIES AIRCRAFT
1. PURPOSE. This handbook provides maintenance inspection notes which can be used for the maintenance support program for certain structural parts of the B-707/720 series aircraft.
2. DESCRIPTION, Maintenance inspection matters on the wing and fuselage are reviewed with a view toward supplementing information currently available.
3. HOW TO GET THIS PUBLICATION.
a. Order additional copies of this publication from:
Department of Transportation Federal Aviation Administration Distribution Unit, TAD-484.3 Washington, D.C. 20591
b. Identify this publication as: Advisory Ci:rcular 20- 76 Maintenance Inspection Notes for Boeing B-707/720 Series Aircraft I= Flight Standards Service Initiated by: FS-320
21 Oct 71
TABLE OF CONTENTS
1. Introduction.
2. Description.
3. Background.
4. General Discussion of Structure Surveillance,
5. Cabin Windows.
6. Versions of the B-707/720 Series Aircraft.
7. Aircraft Station Diagrams.
8. Maintenance Information.
LIST OF ILLUSTRATIONS Figure l, Control Cabin Window Locations.
Figure 2. Passenger Cabin Window Locations.
APPENDICES APPENDIX l, Fundamentals of Airframe Inspection.
APPENDIX 2. Corrosion.
APPENDIX 3. Fatigue Failures.
APPENDIX 4. Nondestructive Testing.
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21 Oct 71 AC 20-76
1. INTRODUCTION. This advisory circular provides maintenance information which can be used, but is not required to be used, by mechanics, repair agencies, owners, and operators in developing maintenance programs, mak ing improvements in existing programs, and conducting inspection and repairs on certain structural parts of Boeing B-707/720 airplanes. The material is based, in part, upon information made available through dis cussions with personnel who have maintained these types of airplanes for thousands of hours of time in service. The intent of the circular is to impart some of this knowledge to other interested persons so that it is not lost.
2. DESCRIPTION. This circular contains guidance material for performing maintenance on wing, fuselage, and empennage structure. The information has been derived from service experience. It does not comprise a full and complete maintenance program for the subject aircraft but should be considered as supplemental maintenance data. Included in the circular, are diagrammatic sketches of the control cabin windows and passenger cab in windows. In addition, there is a listing of selected maintenance difficulties which have been reported since 1964.
3. BACKGROUND.
a. Older Aircraft. The administrator has realized that several dif ferent types of transport aircraft are being phased out of service by some airlines because of the availability of newer equipment.
Such older aircraft are being purchased by other operators who may not be familiar with the scope of required maintenance and the means which have been used to keep the aircraft in a safe condition.
b. Maintenance "Know How. 11 Since maintenance "know how" is not trans ferred with the aircraft, the new operator generally goes through a learning cycle before he is able to rapidly pinpoint the important/ critical problem areas of the aircraft. In this respect, identifi cation of known areas where structural problems have been experi enced will help in the preparation of an initial maintenance program by a new operator. It also can serve as a guide to other operators who have not accumulated sufficient service experience to have knowl edge of all the problem areas of the aircraft.
4. GENERAL DISCUSSION OF STRUCTURE SURVEILLANCE.
a. Manufacturer's Service Bulletins. The manufacturer has published service bulletins containing its recommendations concerning the in spection, repair, and modification of aircraft. Most of these bul letins cover known areas; however, some are predictive in nature and have been issued even though no fatigue damage has been identified in the fleet. Because of differences in structural configuration, most service bulletins apply only to certain aircraft. Effectivity is shown in each bulletin. Additional bulletins may be published by the manufacturer, and a service bulletin is available from the Page 1
AC 20-76 21 Oct 71 manufacturer and is updated periodically. (Note:· Al though service bulletins may be referred to in Airworthiness Directives (ADs) issued by the FAA, the bulletins do not supersede any of the requirements of the ADs.)
b. Other Documentation. ~urther, the manufacturer ha• published the fol lowing documents to aid in maintaining the aircraft in serviceable condition:
Maintenance Planning Data Document, Maintenance Manual, Overhaul Manual, Structural Repair Manual, Non-Destructive Test Document.
This documentation is updated from time to time by the manufacturer.
Structural Item Interim Advisories are published by the manufacturer to notify operators of newly found problems which may be of fleet wide significance and may or may not be followed by service bulletins.
c. Maintenance Action. For adequate maintenance of the 707 and 720 structure, every operator should have in his possession and be conver sant with the above documentation, including service bulletins appli cable to his particular aircraft. He should also obtain complete service records from previous <l'ners and become familiar with the structural history of his aircraft, including information on mainte· nance procedures followed, major repairs made, and preventive modifi cations and/o; repair work incorporated per service bulletins. This is essential to carry out the follow-up procedures required, and to avoid unnecessary work where corrective action has already been taken.
Cl) The new operator should contact the manufacturer regarding any areas requiring clarification.
(2) The operator should keep himself informed of new developments and arrange to be supplied with revised and new documentation by the manufacturer. Consultation with the manufacturer and/or more experienced operators should take place from time to time as necessary, to establish which service bulletins have structural significance and when they would best be incorporated.
(3) The maintenance program established by the new ~perator should reflect changes in environment and usage of the aircraft (e.g., shorter flights, intermittent use, etc.).
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d. Problem Areas. Areas with known or predicted problems are presently covered by service bulletins. These areas should remain airworthy with the documented maintenance, inspection, preventive modification and repair procedures, There are other areas that may become criti cal where predictive methods have not been usable. These are of con cern because of the difficulty to observe them regularly, rather than because they are considered suspect and damage-prone. Potential prob lems involve fatigue or corrosion. These areas are further discussed in Section 8.
5. CABIN WINDOWS. During operation at altitude, the B-707/720 series air craft passengers and control cabin spaces will be pressurized. In the event either a passenger or control cabin window "blows out" while pressurized, the aircraft will experience an "explosive" or sudden decom pression. This occurrence could result in a catastrophic accident and underscores the importance of adequate maintenance inspection program for the aircraft cabin windows. This discussion on cabin windows will brief ly highlight maintenance inspections. However, there are other windows such as inspection windows, toilet windows, etc. The manufacturers' maintenance manual should be consulted for applicable details.
a. Control Cabin Windows (5). The control cabin windows (1 through 5) consist of a thick transparent vinyl plastic core sandwiched between an inner and outer tempered glass layer. On the inboard face of Number 4 window, there is an additional vinyl core and a layer of acrylic sheet, Sliding windows are in Number 2 window position.
The other windows are stationary and bolted to the airframe. (See Figure 1).
(1) Inspection. Examine each window for:
(a) Scratches, cracks, delaminations, discoloration of panes, and chipping.
(b) Binding of sliding windows, positive locking and locking mechanism release, seal deterioration, wear of bushings and guides.
(c) Loose or missing window frame fasteoers.
(d) Rain repellency of windows.
(2) Scratched Windows.
(a) Inner pane scratched. Inner pane scyatches may contribute to a window failure under cabin pressure and heat,
(b) Outer pane scratched. Scratches on the outer pane reduces pane strength under normal cabin pressurization conditions.
Any scratch causes stress concentration, Page 3
AC 20- 7621 Oct 71 · NO., RH (EYEBROW WINDOW) NO, 1 RH (FORWARD WIHOSHIELD) NO., LH (EYEBROW WINDOW) HO, 3 LH (CREW SIDE wtNDOWl NO. 2 LH (PILOT SLIDING WINDOW) N0,, 1 LH (FORWARD WINDSHIELD) ) FIGURE 1. CONTROL CABIN WINDOW LOOATIONS J?age 4
21 Oct 71 AC 20-76
(3) Cracked panes. Any window with a cracked pane should be replaced.
(a) Inner pane cracked. This pane takes pressurization load and the airplane should not be pressurized if this pane is cracked.
(b) Outer pane cracked. Cracks tmt extend toward the center of the pane or span the pane usually become progressively worse and the pane should be replaced at the first opportunity.
(c) Cracking of vinyl usually occurs in the upper aft corner of the No. 1 windshields. It is a fail-safe, bird-proof core. The window should be replaced at the first opportunity.
(4) Delamination. A delamination is the separation of either pane of glass from the vinyl core. When minor delamination occurs between the inner or outer pane and the vinyl core, the airplane could be flown pressurized. An electrically heated window need not be replaced due to delaminations unless the visibility is restricted or lack of window heating, or the pane is scorched.
(5) Chipping. Chips on the laminated surface of the glass are layers or flakes of glass broken from a surface by excessive stresses. Presence of glass chips on laminated surface should be reason enough for window replacement if airplane is to be pressurized.
(6) Arcing. Arcingu.11 produce hot spots with possible eventual result of a cracked pane.
b. Passenger Cabin Windows. These windows are located between the fuselage frames in the areas where passenger seating is provided.
Passenger cabin windows consist of outer, center, and inner panes.
The outer and inner panes are each capable of taking the full cabin pressurization load. The inner pane is nonstructural. All three panes are acrylic. The emergency exit hatch passenger window in stallations are very similar to those for the outer and center panes of the other passenger windows. (See Figure 2).
(1) Inspection/checks of passenger cabinwindows. Examine the window for:
(a) Scratches, cracks, and crazing of acrylic panes.
(b) Deterioration of seals.
(c) Marks, Scratches, and dents.
(d) Loose or missing fasteners at window retaining brackets.
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21 Oct 71 AC 20-76
c. Criteria for Window Inspections.
(1) Optical and visibility criterion should be established by the operator.
(2) A crack is a fissure perpendicular to the surface which extends completely through the pane. Airplane should not be pressur ized with a cracked pane.
(3) Crazing is a series of small fissures perpendicular to the sur face but not extending through pane. Crazing can be caused by improper pane installation or exposure to certain liquids.
(a) Surface crazing is permissible within certain limits if not a routed edge.
(b) Routed radius crazing of a pane is more serious than in the overall surface. Since routed crazing requires pane removal to monitor the extent of crazing, the pane may as well be replaced.
(4) Delamination is a separation of adjac·ent layers of laminate parallel to pane surface. Manufacturer's maintenance manuals provide recommended delamination limits and should be consulted.
(5) Scratches in routed radius will be impossible to see, unless window pane is removed.
(a) Scratches are JDOSt likely found on the exterior surface of the pane which is exposed to weather and washing and accompanying foreign bodies which inevitably get on that surface.
(b) Scratches are easily detected by visual examination and are critical if they exceed certain limits. Manufacturer's maintenance manuals provide recommended limits on scratch, depth and length and should be consulted. (Figures 1 & 2)
6. VERSIONS OF THE B-707/720 SERIES AIRCRAFT. The basic versions of the B-707/720 series aircraft follow. For a detailed description of each version which includes weight, loadings, performance, intermixing of engines, etc. the applicable FAA aircraft type certificate data sheet as indicated should be consulted. Further, the data sheet describes the type design and sets forth the operating limitations,_and other limitations and information found necessary for type certification of a particular model aircraft as prescribed by the FARs.
21 Oct 71AC 20-76 ··
a.. B-707-100 long body. The FAA type certifit:ate appro·val was issued on September 18, 1958. This aircraft was the first production version and was intended for continental flights but also had capability of over-water.operation. Thia aircraft is powered by four Pratt and Whitney JT3C-6 turbojet engines. (See Type Certificate Data Sheet· No. 4A21.)
b. B-707-lOOB long body. The FAA type certificate approval was issued on March 1, 1961. The aircraft was a further development of the basic B- 707· 100 with four Pratt & Whitney JT3D-l turbofan engine·s .
with similar changes incorporated in the B-720. Included were a new inboard wing leading edge and additional segments (four) of the leading edge flaps. (See Type Certificate Data She~t No. 4A21.)
c. Model B-707-lOOB short body. FAA type certificate approval issued July 25, 1961. Same as B-707-1008 long body, except for shorter fuselage, wing changes, and other associated changes and limitations.
Aircraft is powered by four Pratt & Whitney JT3D-l, or JT3D-IMC6 or JT3D-IMC7 turbofan engines. (See Type Certificate Data Sheet No. 4A21.)
d. B-707-200. FAA type certificate approval was issued on November 5, 1959. This aircraft is identical to the B-707-100 long body but is powered by four Pratt & Whitney JT4A-3 engines. (See ) Type Certificate Data Sheet No. 4A21.)
e. B-707-300 Intercontinental. FAA type ceritificate approval was issued on July 15, 1959. This aircraft is an over-water long range version featuring an increase in wing span and longer fuselage. This air craft is powr-red by four Pratt & Whitney JT4A-5 engines,JT4A-3 engines JT4A-9 engines or JT-4A-ll engines. (See Type Certificate Data Sheet No. 4A26.)
f. B-707-300B·!ntercontinental. FAA type certificate approval issued on May 31, 1962. This aircraft was a further de··.,iopment of the B-707-300 and powered by four Pratt & Whitney JT3D-$ or JT3D-3B turbofan engines. The design changes included new leading and trailing edge flaps plus low-drag wing tips. (See Type Certificate Data Sheet No. 4A26.)
g. B-707-300C Intercontinental. FAA type certificate approval was issued on April 30~ 1963. This aircraft features a cargo or mixed cargo passenger version of the B-707-300B with a forward cargo door (94 inches z-134 inches) and a cargo loading system using pallets and containers. The cargo apace includes the full upper deck and the two lower deck holds. This aircraft is powered by four Pratt & Whitney JT3D-7 en~ines. (See Type Certificate Data Sheet 4A26,) )
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h. B-707-400 Intercontinental. FAA type certificate approval was issued February 12, 1960. This aircraft is identical to the B-707-300 but powered by Rolls Royce Conway MK508 turbofan engines. (See Type Certificate Data Sheet No. 4A26.)
i. B-720 Series Aircraft. The B-720 is essentially identical to the B-707-100 in external outline, main dimension, aerodynamic design and control systems although the B-720 is completely different from a weight and structural strength design standpoint. However, most B-720 aircraft parts and repairs are interchangeable with the B-707/720 aircraft. The B-720 aircraft features a wing leading edge change resulting in an aerodynamic variance which improved the air craft performance. (These aforementioned changes were incorporated in the B-707-lOOB version.) The B-720-720B series aircraft have several different models due to different installations of equip ment, interiors, airplane flight manuals, etc.
(1) B-720. FAA type certificate approval was issued on June 30,1960.
The basic aircraft is powered by four Pratt & Whitney JT3C-7 turbojet engines or JT3C-12 engines. (See Type Certificate Data Sheet No. 4A28.)
(2) B-720B. FAA type certificate approval was issued on March 3, 1961. This aircraft is powered by four Pratt & Whitney JT3D-l or .IT-3D-3 turbofan engines. (See Type Certificate Data Sheet No. 4A28.)
7. AIRCRAFT STATION DIAGRAMS. The Manufacturers Structural Repair Manual should be consulted for aircraft station diagrams.
8. MAINTENANCE INFORMATION. The requirements for structural surveillance of older airplanes vary greatly.
a, Factors that should be considered include:
(1) Basic structural differences of the various models.
(2) Production changes made to correct structural problems.
(3) Production changes made for weight reduction.
(4) Preventive modifications made per service bulletins.
(5) Repairs made per service bulletins.
(6) Repairs made per Structural Repair Manual.
(7) Incorporation of special repairs, correcting structural problems.